Periodic Reporting for period 1 - SELENDIS (SELENDIS: A novel detection strategy to discover light Dark Matter)
Okres sprawozdawczy: 2019-04-01 do 2021-03-31
Ordinary matter - which includes stars, galaxies as well as the human body - only accounts for 20% of the total mass of the Universe. The missing mass consists of so-called Dark Matter (DM) which, while invisible, betrays its presence through gravitational effects on its surrounding environment. For the past several decades, DM particles have been extensively sought after but yet keep eluding detection. Why? There are now compelling reasons to believe that these particles are lighter than first anticipated. As a result, their discovery requires energy detection thresholds and background discrimination capabilities beyond state-of-the-art detector performance. The goal of this research project is to design, develop and operate a novel direct detection technology called SELENDIS (Single ELEctron Nuclear recoil DIScrimination). SELENDIS consists of small Ge crystals equipped with thermal sensors and Al electrodes, operated at a high voltage (100 V) as a charge amplifier. Its originality comes from achieving the two key experimental requirements for light DM discovery: sub-100 eV detection and discrimination energy thresholds, with the sole measurement of the thermal signal.
Most of the R&D has been focused on designing and fabricating both SELENDIS detectors and a dedicated laser-based calibration set-up. When envisioning detectors capable of detecting single electron/hole pair equivalent energies, even calibration devices need to be thought of and adapted to experimental constraints. In our case, detectors are placed at 20 mK (that is -273,13 °C) in a cryostat, under vacuum, separated from room temperature by screens at various temperature stages following a ‘Russian doll’ architecture. We have developed a table-top laser-based system using a pulsed 1550nm laser-diode equipped with an integrated photodiode to allow for a continuous monitoring of optical power during calibration measurements. Remotely controlled variable attenuators confer a dynamic range in optical power allowing for the simultaneous calibration of 4 different detectors at energies ranging from 10’s of keV down to a single electron hole pair. A custom made flange permits the optical fibers feedthrough to the inner part of the cryostat where unjacked fibers of only 125 um diameter travel across the temperature stages from 300 K to 20 mK to reach the detectors. The acquisition electronics and software have been adapted to record timing and optical power information of laser-diode induced events and disentangle them from background events. This whole setup was successfully tested and validated down to energies as low as the detector trigger thresholds.
The detector design phase extended over more than a year. The optimal and final detector configurations resulted from studies based on : 1) finite element simulations of electric field lines in various detector geometries 2) Comparative study of the performance of prototypes and existing detectors with different electrode designs. From an initially envisioned array of two 30-g Si and Ge crystals, we finally opted for an array of six 3-g Ge crystals. We built, assembled, and instrumented four detectors with NTD thermal sensors. Two of these detectors were operated as pure calorimeters in a first step towards validating the phonon resolution objective of 15 eV (rms), which while long-delayed due to cryogenic issues was eventually exceeded with 13 eV. Two other crystals were equipped with aluminum electrodes lithographed in a grid scheme, known to withstand a high voltage without leaking. The two remaining crystals were optically polished in preparation of the next generation of SELENDIS detectors with electrodes - made of Sapphire wafers with a thin film of Al - separated from the crystal by a vacuum gap, allowing for voltages even greater than 100V to be applied.
Although the design, fabrication and first performance tests were all successful, the detectors equipped with electrodes were only ready to be operated at the very end of the fellowship. This delay, mostly due to both cryogenic issues and the Covid pandemic, postponed the detector applications initially planned to occur during the fellowship.
The detector design phase extended over more than a year. The optimal and final detector configurations resulted from studies based on : 1) finite element simulations of electric field lines in various detector geometries 2) Comparative study of the performance of prototypes and existing detectors with different electrode designs. From an initially envisioned array of two 30-g Si and Ge crystals, we finally opted for an array of six 3-g Ge crystals. We built, assembled, and instrumented four detectors with NTD thermal sensors. Two of these detectors were operated as pure calorimeters in a first step towards validating the phonon resolution objective of 15 eV (rms), which while long-delayed due to cryogenic issues was eventually exceeded with 13 eV. Two other crystals were equipped with aluminum electrodes lithographed in a grid scheme, known to withstand a high voltage without leaking. The two remaining crystals were optically polished in preparation of the next generation of SELENDIS detectors with electrodes - made of Sapphire wafers with a thin film of Al - separated from the crystal by a vacuum gap, allowing for voltages even greater than 100V to be applied.
Although the design, fabrication and first performance tests were all successful, the detectors equipped with electrodes were only ready to be operated at the very end of the fellowship. This delay, mostly due to both cryogenic issues and the Covid pandemic, postponed the detector applications initially planned to occur during the fellowship.
When I started my fellowship, EDELWEISS was acquiring data at the Laboratoire Souterrain de Modane (LSM) with a large number of detectors of different masses ranging from 32g to 800g and a variety of electrode schemes and thermal sensors. I naturally got interested in studying their performance as I worked on the design phase of SELENDIS. One detector in particular triggered my interest as it seemed to withstand a high voltage operation. With this detector, we managed to obtain a 0.5 e-/h+ pair (rms) from a 40 eV phonon baseline by operating it at 78 V. I carried out an in-depth analysis of the data acquired in April 2019 and set the first Ge-based constraints on sub-MeV DM particles interacting with electrons, as well as on dark photons down to 1 eV/c². These results were not only competitive with other state-of-the-art experiments, these allowed to set new limits on the kinetic mixing of dark photon DM in a so far unconstrained parameter space region, demonstrating the high relevance of SELENDIS-like detectors for the search of exotic light DM. Together with the EDELWEISS Collaboration, we published the associated results in Phys. Rev. Letters, a specialized peer-review Journal with a high impact factor.
These groundbreaking results were obtained with a detector that had not been designed for a high voltage operation and in spite of resolution performance a factor 2 below those envisioned for SELENDIS. This is very promising for the physics reach of SELENDIS detectors.
These groundbreaking results were obtained with a detector that had not been designed for a high voltage operation and in spite of resolution performance a factor 2 below those envisioned for SELENDIS. This is very promising for the physics reach of SELENDIS detectors.